1. Field of the Invention
The invention concerns a method for medical imaging as well as a corresponding medical imaging device.
2. Description of the Prior Art
Medical imaging is used for imaging, for implementing radiological examinations. In a radiological examination a subject (in particular a patient) is exposed with radiation. Electromagnetic beams or particle beams, for example x-rays or electrons, can be used for the exposure. As used herein, “an exposure” means radiograph of the patient.
Predominantly due to different local material properties of the tissue structures located in the beam path, the incident radiation is affected to different degrees in its passage through the body of the patient. In particular, different tissue structures have different radiation attenuation properties. This causes the beams passing through the body of the patient to be attenuated to different degrees. A detector signal corresponding to the intensity I of the attenuated beams can be generated by an appropriate detector. Such an attenuation is frequently defined as a logarithm of the ratio of the intensity of the attenuated beams to the intensity of the primary radiation (˜I/I0). The different tissue structures can be visualized as a projection using an attenuation distribution.
In tomography (for example x-ray computed tomography, also called CT for short) a plane of the subject (and in particular of the patient) is systematically radiographed from different directions, with the effect on the beams that are used (in particular their attenuation) is detected for each direction. Overall, a number of projections are thus acquired by means of which the attenuation distribution in the observed subject plane (and thus ultimately a spatial image exposure of said subject) is acquired.
The dose applied in such radiography of the patient with electromagnetic radiation is consistently the subject matter of intensive and critical discussions. The dose represents a measurement for the absorption of ionizing radiation (for example x-rays) by the exposed subject. In medicine, radiologically evaluated dose quantities are defined (for example in the form of an organ dose) to account for different radiation risks for various radiation types and for various tissue types.
The attempt is to keep the applied dose as low as possible proceeds according to the ALARA principle (ALARA=As Low As Reasonably Achievable).
Recently, current-modulated automatic dosing has increasingly been used, in particular in imaging by computed tomography. For example, such an automatic dosing can be learned from DE 102 38 894 A1 as well as in the article “Dosisreduktion durch strommodulierte Dosisautomatik bei der MSCT: Vergleich von Messung und Rechnung” (“Dose reduction via current-modulated automatic dosing in MSCT: Comparison of measurement and computation”) by U. Lechel, C R Becker, G. Langenfeld-Jäger and G. Brix from Fortschritte auf dem Gebiet der Röntgenstrahlen und der bildgebenden Verfahren (Advances in the Field of X-rays and Imaging Methods), 2007 (179). An essentially constant image quality within an image exposure (in particular in the sense of a noise portion in the detector signal) is sought with such an automatic dosing. In order to achieve the constant image quality, the tube current of the computed tomography apparatus (and therefore the radiation power of its x-ray radiator) is frequently adapted to the locally different radiation attenuation relationships in the patient. The noise component in the image exposure is normally higher the fewer radiation quanta (in particular x-ray quanta) that are registered at the detector. It can also be stated more simply that the noise component is lower (and therefore the image quality is higher) given a high tube current than given a low tube current. Given a constant x-ray current, the noise component depends in particular on the existing radiation attenuation relationships in the patient. In a first approximation, the greater the radiation attenuation property, the higher the consequent noise component in the image exposure. The applied dose in turn linearly depends on, among other things, the x-ray current-time product. Essentially, by x-ray current modulation controlled by the image quality, as high a dose is applied as is necessary in order to achieve a noise component in the detector signal that is established corresponding to the predetermined image quality.